Vector Plotting

Vector plotting uses two grid fields to accommodate the X and Y components of the
vector. For the most part, this is the U and V wind component grids. To visualize the
vector grids, there are the following plot types:

Plot Type

Description

vect

Plot wind vectors

barb

Plot wind barbs

strm or stream

Plot streamlines

Vector Plots

Vectors start at the grid location and point in the direction the wind is blowing to.
The length of the vector is dependent on its speed. All vectors are scaled so that the
largest wind vector is equal to one grid spacing.

When looping wind vector plots, it should be noted that since the wind vectors are
scaled to the highest wind speed, the vector lengths for the same wind speed could change
from one frame to the next. Use the con_base
resource to lock a specific length to a specific speed. The contour base value sets a wind
vector of that speed to one grid spacing, thus locking in a length to speed relationship.
Remember to pick a value close to the maximum speed or the vectors may be too large or too
small.

The plot_scale resource will the
vectors if they are too small to read or if the grid spacing is too small as is the case
with most of the model data. If this is the case, it is recommended not to plot all
wind vectors. The wv_skip=xskip[,yskip]plot
parameter force the plotting to skip every skip vectors. Then it might be necessary
to set the plot scale to the skip factor in order to make the vectors more readable. A
sample is:

Vector Bar

To label the wind vector plot, a vector bar can be plotted. By default, only the
maximum speed is displayed in the lower right corner of the plot. A vector bar is an arrow
representing the largest arrow/vector on the plot along with its value. This can be
enabled with the "vbar" plot parameter.

Wind Barbs

Wind barbs start at the grid location and point in the direction the wind is blowing
from. This uses a standard wind barb where a full barb represents 10 for whatever
parameter is being displayed. A sample is:

Streamlines

Streamlines show the full continuous flow of the vector field that is often used to
display winds. The streamline algorithm starts a streamline out of the center of each grid
box. The streamline continues until it reaches a maximum length, reaches a grid box with
too many streamlines already in it or when the streamline makes more than a 90 degree turn
in either direction.

The length of the streamline is set so that it is roughly the length from one end of
the domain to the other. The maximum length can be controlled by the plot_scaleparameter. The number of
streamlines within a grid box is controlled by the con_base resource. The streamline algorithm
automatically doubles the grid for this computation. The default is one streamline per
grid box but this is roughly 2 since the grid is doubled. Setting the contour base to 2
would double the number of streamlines on the plot.

In order to thin out the streamlines, the wv_skip plot parameter can be invoked.
Setting this to 2 would start streamlines out of every other grid box. Since model grids
are so dense, it is recommended to set the vector skip parameter to make the streamlines
more readable.

The arrows on the streamlines can become rather small to view on large plot domains.
There is a streamline scale factor plot parameter "strm_sc=scale"
which is used to determine the distance between arrows and the size of the arrows.

U, V Component Computation

There are two methods for computing U and V wind components. The first method is the
compute U as the easterly component of the wind and V as the northerly. This is the
default for WXP if the U and V components are individually computed using the uwnd
and vwnd variable specification. The second method is compute U and V relative to
the grid where U is the component to the right on the grid and V is towards the top of the
grid. This can be specified with either the uwndg or vwndg variable
specification. A composite vector variable called wind is available in most
applications, which are the grid relative winds.

The default for the vector plotting module is to use grid relative winds but this can
be forced with the uvgrid plot parameter. If the U and V grids are east/west
relative, use the uvew plot parameter to make sure the vectors are properly aligned
to the domain.

Plot Attributes

Plotting attributes affect how the vectors will be plotted. The types of attributes
available include color, style, and width.

Resources

The color, line width, text size and scale factor can be set with the following
resources:

Other attributes can be changed when specifying the variable or in the variable file.
When specifying a variable, the following syntax is used:

[time:level]varname[[units[:attributes...]]]

The time and level parameters are the same as can be specified with the time and level
resources. These are often specified with these resources and therefore are optional. The
units and attributes are surrounded with brackets "[ ]".

Units

The input data will have a specific set of units that can be modified by specifying the
units within the brackets. For example, to change the temperature to Celsius:

The attributes are listed after the units. Multiple attributes can be specified
separated by colons ":". For example, to plot temperature in C,
using blue line contours:

-va=wind[knt:vect:co=blue]

Here is a list of available attributes to specify:

Attribute

Description

type

The type of plot: vect, barb, stream

mx=max

Specifies the maximum value in the contour plot. All values greater than
this value will not be plotted.

mn=min

Specifies the minimum value in the contour plot. All values less
than this value will not be plotted.

co=color

Specifies the color. This is the color name from the color table.
If this is the first attribute, the "co=" may be omitted.

wi=width

Specifies the width of lines. This includes lines used in text and
markers. The default value is 1.0.

st=style

Specifies the style of lines. Possible values are:

sol - solid lines (number 1)

dsh - dashed lines (number 2)

lsdsh - long short dashed lines (number 3)

llsdsh - long, long, short dashed lines (number 4)

dot - dotted lines (number 5)

# - a number corresponding to the above styles

sc=scale

Specifies the scale factor. This will scale vectors and barbs and scales
the maximum length of streamlines. The default value is 1.0.

ss=scale

Specifies the streamline scale factor. Used to determine the distance
between arrows and the size of the arrows. The default is 1.0.

cb=base

Specifies the contour base which is used to determine how many
streamlines go through a given grid box. The default value is 1.0.

in=intrvl

Specifies a contour interval that is used to color vectors, barbs and
streamlines.

sk=skip

Specifies the wind vector skip factor. The default is 1.

vbar

Specifies to plot the vector bar

Color based on Value

The color of the vectors, barbs and streamlines can be changed based on value of the
speed. This uses the same mechanism that the color fill contours use. If a contour
interval (con_interval resource) is
specified, the data will be colored and an optional color bar can be displayed. The
minimum for the color fill attributes is 0.